The present invention is generally directed to ink jet printers. More particularly, the invention is directed to operating an ink jet print head within a particular power regime to optimize ink nucleation.
Thermal ink jet printing involves providing electrical signal impulses to resistive heaters to generate heat, and transferring the heat into adjacently disposed amounts of ink. The heat transferred into the ink causes the ink to nucleate, thereby forming a vapor bubble which propels a droplet of the ink through an adjacent nozzle and onto a printing medium. A number of factors affect the quality of the images produced by a ink jet printer, such as characteristics of the resistive heaters, properties of the ink, and the geometry of the nozzles. All of these factors affect how precisely the ink droplet ejected from the nozzle is placed on the printing medium. Since the print medium and the print head are typically moving with respect to each other as the ink droplet is ejected, the velocity with which the ink droplet is expelled from the nozzle influences the placement of the droplet on the paper. Thus, to maintain good image quality, it is imperative to maintain a stable and predictable droplet velocity.
Therefore, an ink jet printer is needed which maintains stable and predictable ink droplet velocity.
The foregoing and other needs are met by an ink jet printer that forms printed images by ejecting droplets of ink at a stable velocity onto a print medium. The printer includes an ink jet print head having a plurality of nozzles through which the droplets of ink are ejected. The print head includes a heater chip having a plurality of heating elements, each of which is associated with a corresponding one of the plurality of nozzles. Each heating element transfers heat into adjacent ink at a predetermined rate of heat transfer sufficient to maintain the stable velocity of the droplets of ink, where the predetermined rate of heat transfer is accomplished when a predetermined minimum power level is applied to the heating element.
Each heating element includes a heater resistor and a protective layer adjacent the heater resistor. Each heater resistor has a heater resistor thermal capacitance value, a heater resistor area, and a heater resistor thickness. Each heater resistor is operable to provide a predetermined minimum power density per unit area when the predetermined minimum power level is applied to the heater resistor. The protective layer has a protective layer thermal capacitance value and a protective layer thickness. The heater resistor area multiplied by a sum of the heater resistor thickness and the protective layer thickness represents a heating element volume. Each heating element is operable to provide a predetermined minimum power density per unit volume within the heating element volume when the predetermined minimum power level is applied to an associated heater resistor. According to preferred embodiments of the invention, the predetermined minimum power density per unit volume is determined by the predetermined minimum power density per unit area divided by the sum of the heater resistor thickness and the protective layer thickness.
The printer includes a power supply coupled to the plurality of heater resistors for providing the predetermined minimum power level to the heater resistors. In preferred embodiments of the invention, the power supply provides the predetermined minimum power level sufficient to generate the predetermined minimum power density per unit volume of at least about 1.5xc3x971015 watts per cubic meter. By providing a power density per unit volume of at least about 1.5xc3x971015 watts per cubic meter in the heating elements of the print head, the invention ensures stable droplet velocity and bubble nucleation quality, thereby enhancing the quality of the printed images.
In another aspect, the invention provides a method for printing with an ink jet printer by ejecting droplets of ink at a stable velocity onto a print medium. The method includes providing a thermal ink jet print head having a plurality of nozzles through which the droplets of ink are ejected. The print head has a heater chip which includes a plurality of heating elements, where each heating element is associated with a corresponding one of the plurality of nozzles. Each heating element in the print head includes a heater resistor having a heater resistor area and a heater resistor thickness, and a protective layer adjacent the heater resistor which has a protective layer thickness. The heater resistor area multiplied by a sum of the heater resistor thickness and the protective layer thickness represents a heating element volume. The method further includes providing a power density per unit volume within the heating element volume of at least about 1.5xc3x971015 watts per cubic meter.
In yet another aspect, the invention provides a method for operating a thermal ink jet print head to provide an optimum power density per unit area at a surface of an ink heating resistor within the print head. The method includes providing the thermal ink jet print head having a plurality of nozzles through which droplets of ink are ejected. Within the print head is a heater chip which includes a plurality of heating elements, where each heating element is associated with a corresponding one of the plurality of nozzles. Each heating element includes a heater resistor having a heater resistor thickness tR and a heater resistor surface area, and a protective layer adjacent the heater resistor which has a protective layer thickness tP. The heater resistor surface area multiplied by the sum of the heater resistor thickness and the protective layer thickness represents a heating element volume. The method further includes providing a power density per unit area PDA on the heater resistor surface area according to:
PDA=PDVxc3x97(tR+tP),
where PDV represents the power density per unit volume within the heating element volume, which is at least about 1.5xc3x971015 watts per cubic meter.